Sight-testing apparatus



July 28,1936. R. E. REASON 2,049,222

' SIGHT TESTING APPARATUS Filed June 12, 1935 2 Shets-Sheet 1 qwmroie 7W HWTM L; ATrbR/vn July 28, 1936. R. E. REASON SIGHT TESTING APPARATUS Filed June 12, 1955 2 Sheets-Sheet 2 M/VE/VTOI? A, A TTOR/Vfl Patented July 28, 1936 S ATES AT a 'l OFFIQE SIGHT-TE STING APPARATUS land Application June 12, 1935, Serial No. 26,124 V In Great Britain June 23, 1934 7 Claims;

The present invention relates to sight-testing apparatus or" the kind comprising a projecting system for forming upon the retina of a subjects eye an image of a target, and an observing sys- 5 term for viewing the retina and the image formed thereon, .and particularly to apparatus of this kind adapted to measure the refraction of the eye, wherein for example, the aXes of the projecting and observing systems are separate at the 10 pupil and intersect, by adjustment, at the retina.

Apparatus of this kind is described in the specificatiens of BritishLetters Patent Nos. 413,029, 43.9,,ft61,.430,836, and 432,727., filed January .24, 19-33., January 18,, 1934, January 29, 1934, and

1 February 3', 1934, respectively.

Insotder to determine the refraction in various orientations, ror .in' order to determine the axis of astigmatism of the eye, the apparatus has been made; rotatable about the axis of the eye and it Z03 hasv also been proposed to obtain an equivalent efiect by inserting a Dove reversing prism in the apparatus and rotating this prism instead of the whole apparatus. Furthermore in order to measure the refraction accurately the axis of the apparatusmust be aligned with respect to the centre of the pupil and for this purpose the apparatus 11881138811 made adjustable about two perpendiculanilateralaxes'both of which are perpendicular to ltheeaXiS of rotation of the apparatus.

' Such: a prismatic arrangement suffers from several disadvantages however. In the first place since a Dove prism comprises one reflecting and twai -"retracting surfaces and the light enters and leaves theprism at substantial angles to the entran tand emergent retracting faces, aberrations are introduced at these faces. Secondly, in apparatus comprising means for observing the subjects'pupi-l-to facilitate alignment, the image of the pupil, when the observing system is adjustedalong an axis-inclinedto the lateral axis unless the plane of reversal "happens to be parallel to the lateral axis; in general, therefore, it is difiicult for'an observer to decide in which lateral -di--- rection-t-he instrument should be displaced in orabout onef the lateral axes, appears to move According to the present invention sight-testing apparatus comprises a projecting system for formingon the retina of a subjects eye an image of a target and an observing system for viewing the retina and the image formed thereon, where- 5 in there is provided a rotatable reversing member which is common to the projecting and observing systems and which has three or five surfaces at which light is reflected, the elements of the ob-' serving and projecting systems on the target side of said rotatable reversing member being disposed on an axis which is parallel to but ofiset with respect to an axis upon which the member and the elements on the eye side of the member are disposed. The reversing member preferably has the form of an internally reflecting prism, although separated externally reflecting surfaces may also be used.

The invention will now be described with reference to the accompanying diagrammatic draw- 0 ings, in which Figure 1 shows the optical elements of one form of sight-testing apparatus arranged in accordance with the present invention, Figure 2 is a view, taken on the line AA of Figure 1, of a mask shown in Figure -1. Figure 3 shows an alternative arrangement of the elements I to 8 inclusiveyof Figure 1, Figure 4 illustrates the field of view seen in the instrument of Figure 1, Figure '5 is a plan view of a reversing prism shown in Figure .1, Figures 6. and '7 are front and side lelevations respectively of .a reversing prism which may :be used in :place of the reversing prism 28 shown in Figures 1 and .5, Figure 8 is .a view of a purely reflecting device which may be used in place of the reversing prism 28 of Figure 1, and Figure 9 is a view illustrating the formation of ghost reflections at the surfaces of a lens and reversing prism shown in Figure 1.

Referring .now to Figure 1, light from a source I is condensed upon a target 2 in the form of a slit, by means :of a lens 3. Light diverging from each point in :the target 2 passes through one :of two slots 4 and 5 (see Figure 2) disposedat right angles in a plate 6 which may be moved, in a direction bisecting the angle between the slots, by means of a screw 1. Beneath the plate 5 is a second ,plate 8 having an aperture '9. The plate 8 may be moved bya handle It), so as to uncover either of the slots l and 5. Light passing through the slot 5 is deviated at a reflector ll (Figure 1) and is focused, by lens 12, to an image I3, and a further image of 13' is formed by lens M and eyelens 1'5 at l6 upon the retina of the subject's eye. The reflector l l comprises two perpendicularly disposed radial arms HA and Il-B (see Figure 2) on the subjects pupil through which light passes towards the retina. If desired the plates 6 and 8 may be disposed, as shown in Figure3, between the target 2 and the source, I and an image of the plates formed substantially at the reflector,

I I by means of a lens II. This arrangement may be found preferable since the members of the de limiting device and the reflector I I 'may then in effect be superimposed at the image of the pupil.

The lenses I2 and I4 are arranged with their focal planes at the reflector II and the eye-lens I5 respectively sothat the light imaging the pupil is parallel between'them and an image of the eye-lens I5 is formed coincident with the re- The lenses I 5 and I4 together form an image of the retina at I3 and, in the absence of lenses 3!! and 3i, the functions of which will be explained later, lens I2 forms an image of IS at I8. The light rays forming the image I3 are divided, partially reversed and observed (in the manner described inthe above-mentioned co-pen'ding applications) with the aid'of the components I9 to 26 inclusive, 22 being a reversing prism'in one of the divided beams and 25 and 26 being a .microscope eye-piece.

Thus images of the retina are formed at l 3 and I8 and a partially reversed image at 24, this last image beingviewed in the eye-piece 25, 26. In order to focus an image of the target? accurately on the retina the whole of the system to the left of the lens I4 is mounted for movement along an axis 2? so that the image I3 of the target 2 may be moved towards or awayfrom the lens I4,-the' {projecting system being in no way disturbed by this movement since parallel light passes between the lenses I2 and M. The refraction of the eye is determined by measuring the distance through I which the image IS'of the target has to be moved in order to focus it on the retinas. Measurements in various orientations are achieved by rotating a reversing prism 28 about an .axis 29 which is parallel tobut offset'with respect to axis 27 and which passes through the centre of the eye-lens I5, rotation of this prism having the same efifect as if the subjects eye were'rotated about its centre, whilst the whole optical apparatus were kept fixed. Y

The axis of the reversing member, whether it be the prism 28 or some other system of reflectors, is taken to be that line the-reflected image of which emerges without a shift or r change of direction. I

In order toobserv the subjects pupil and alignthe observingsystein with respect thereto an auxiliary pupil-sighting lens SI and a field lens 39 are swung into the, observing system as described in British Patent 430,836, filed January 29, 1934, these'lens being adapted to form an image of the reflector II and hence of the pupil at the positions I8 and 24, this composite image being viewed in the eye piece 25, 26. The horizontal arm of the reflector II is roof-shaped on Mapp r s d oe eeeqi h r 1 gl v to reflect light from the target 2 forwards towards the subjects'eye, and the other face serving to scatter or reflect part of the light passing through the slot 5 backwards towards the eye-piece 25, 26. On looking through the eye-piece with the lenses 30 and 3! in position, the observer therefore seesan image 32 (Figure 4) of the pupil of the eye as a dull'red disc illuminated by light scattered back from the retina, the arms HA and MB of the reflector II with which it has tobe centered, apparently superimposed, in it, and a small bright patch 33 of light on the upper side of one arm which indicates the exact position on the pupil at. which the target beam is passingthrough it,"

this position can be varied'as already described by adjustment of' the plate 6.

A plan view of the prism 28 is shown in Figure 5,

and it will be seen that the prism comprises an odd number of reflecting faces (in this case three) and two refracting faces which are disposed approximately normally to the axis of theprism.

Thus little or no abberation arises at the two ree fracting faces of the prism.

Another form of prism which may be used in place of the prism 28 is shown in Figs. 6 and 7.- It comprises two refracting and five reflecting surfaces. Light enters the prism normally in the direction of the arrow,is reflected in turn at the plane surfaces a, b, c, sufiers two reflections 'at' the V surfaces oh and d2 and then emerges from the prism normally. This form of prism has the advantage that none of the surfaces need be metallized but is more expensive toimake than the prism 28.v Alternatively a device of the kind 7 shown in Figure 8, and comprising three plane mirrors c, J, g, having reflecting surfaces h, j, i,.re-

spectively, may be used in place of the. prism 28 of Fig. 1.

Referring again to Figure 1, great difficulty has been experienced in eliminating ghost reflections from theelements I2, 28, and I I;'it is necessary to eliminate first order'ghosts completely; second order ghosts do not arise, but even third order ghosts must be thrown wellout of focus. Since the illuminated-area of the pupil is delimited at a place where the light isdivergingfrom each point in the target ghost reflections off thelens I 2 are readily thrown outside thefield of the instrument, the first wghost for example proceeding along the path CG, as shown in Fig- Fig.9. I ,1 1

First order reflections of the faces of the prism- 28are returned through the lens I2 and focussed: by auto-collimation in thetplane 0f the reflector II so that they could, if desired, be eliminated by.

placing in the plane of I I ,an opaquemask- H which is the mirror image of .5 Alternatively I such a mask could be placedat'anyimage of the reflector II lying to the left of thereflector it-' self. This methodis somewhat critical however,

because small departures from normality to theaxis of the faces of the prism 28 throw the ghosts.

It may,v therefore, be found off such a mask. preferable to arrange the lens I2 close to the first face 3 3 of the prism (as shown in Figure 9) in; which case the reflections from this face, which pass through II, the mirror image of I I,=are.

thrown to one side of the field at I9 along the,

path DEF for. example, and are not seen in the eye-piece of the instrument.

, Thisarrangement does not, however, dispose of the ghostreflection at the rear face 35. of the prism 28, which, ifthe face were normal to the axis, would proceed along the dotted path and enter I8. 1Iowif the image I3 be formed ac'ls tually at the rear face 35 the latter can be tilted considerably without introducing aberrations affecting the retinal image, so that the ghost reflection can easily be thrown tc'one side of the field, butin'this case scattered light due to dust on the re'ariace 35 then becomes very apparent and may seriously diminish the efficiency and accuracy o'f'the instrument. It isfound, however, that if the rear face 35 be moved away from the image l3 a position is reached at which the scattered lightis insuflicie'nt to spoil the observations but at which the face is still sufficiently close to the image to permit the. face to be tilted sufliciently to'avoid the ghost without introducing appreciable aberration-s. Ultimately a position a is reached where the beams ,36 and 31 entering and emerging from the prism respectively are just completely separated, so that none of the light scattered fromthe face 35 can enter the observing system.

.shown in this figure, therefore, the front face 34 is disposed normally-to its axis 29 and closetothe lens l2, whilst the rear face 35 is inclined' to the axis suificiently to throw ghost reflections to one side and is at a distance from the image 13 suflicient to avoid the introduction of errors due both to scattered light and to aberrations at the surface 35. The necessity for an opaque mask at the reflector II (or at an image thereof) is thus avoided, with the result that practically the whole of the field of view (see Figure '4) between 12 oclock and 9 oclock is unobscured. Consequently the radial distance between the centres of the area of the entering and emerging beams at the subjects pupil, and also the size of the emerging area may be increased, giving ajconcurrent increase in sensitivity=and'brightness.' Referring to Figure 4 this radial distance is that between the centres of the areas 33 and 38.

Finally it is found that in order to avoid ghosts off the lens 14 the image I3 must be formed substantially only on one side of the axis 29. At the same time it is found diflicult to place a target of satisfactory length so that its image in the reflector ll lies wholly on one side of the axis 21 of lens l2, because the lens I2 can be readily freed of ghosts only by mounting the target so that its image in the reflector ll lies symmetrically about the axis 21, or extends towards the same side as the image I3. Furthermore the image of the reflector ll must be rotated by the prism 28 concentrically round the pupil of the eye the axis of which is coincident with that of the lens I4, that is, the image of the point of intersection of the axes 21 and 21' must be centred on the pupil.

These conditions are simultaneously satisfied by mounting the lens l4 and prism 28 on the axis '29 of the eye and the elements to the left of the prism 28 upon the axis 21 which is parallel to axis 29 but sufficiently offset therefrom to ensure that the image I3 of the target lies sufliciently to one side of the axis 29.

Thus the image I3 is suitably displaced and, in addition, because of the reflector II and pupil 15 are at the principal focal planes of the lenses l2 and M respectively, the image at the pupil of said point of intersection remains accurately centred thereon. The axis of the prism 28 and the axis 29 are coaxial so that the illuminating beam rotates concentrically around the lens [4 and the pupil.

It will be noticed that since both the observing and target beams pass through the prism 28, me-

chanical errors in the location of the prism affect both beams equallyand do not introduce errors into the measurements. Furthermore, the target 2, reflector H and prism 28, the relative position of which determines the accuracy of the readings, are moved bodily together for making measurements, so that again errors in the motion affect both beams equally and do not introduce errors into the readings.

In cases where provision is made for sighting on the pupil, for example as described in British Patent.430,836. filed January 29, 1934, the diffipulty, which has been mentioned earlier, of perceiving in which direction and about which of the lateral axes of adjustment the observing System must be moved in order to secure alignment with the pupil is eleminated by providing aradial line, arrow or similar pointer upon a plate 39 (Figure 1) rotatably mounted in the observing system at an image of the pupil. The plate 39 is coupled by suitable gearing to the reversing prism 28 and arranged to rotate in the :same direction as, but at twice the speed of, the prism. The pointer may have a second radial line preferably of different appearance arranged at right angles thereto and may be illuminated by asuitably disposed lamp or may be self-luminous. With this arrangement whatever the angular position of the prism 28, the two lines of thepointer will indicate respectively the direction in which n the observed image of the reflector 11 will move when the instrument is adjusted about each -.of the lateral axes.

The plate 39 may also be inscribed witheircles to facilitate the centering operation. These circles and a pointer in the form of a single arrow are seen in the eye piece somewhat as shown in Figure 4, 40 being the concentric circles and 4| the arrow.

I claim:

1. Sight-testing apparatus including a target, a projecting system of optical elements for forming an image of the target upon the retina of a subjects eye, an observing system of optical elements for viewing the retina and the image formed thereon, and a rotatable reversing member which is common to the projecting and observing systems and which has an odd number not less than three and not greater than five of reflective faces, elements of the observing and projecting systems on the target side of said rotatable reversing member being disposed upon an axis which is parallel to but ofiset with respect to an axis upon which the member and elements on the eye side of the member are disposed.

2. Sight-testing apparatus including a target, a projecting system of optical elements for forming an image of the target upon the retina of a subject's eye, an observing system of optical elements for viewing the retina and the pupil of the subjects eye, a delimiting device, for selecting the area of the pupil through which light enters the eye, disposed substantially at an image of the pupil in light diverging from each point in the target, and a rotatable reversing member which is common to the projecting and observing systems and which has an odd number not less than three and not greater than five of reflective faces, elements of the observing and projecting sys tems on the target side of said rotatable reversing member being disposed upon an axis which is parallel to but offset with respect to an axis upon which the member and elements on the eye side of the member are disposed.

I35 Sight-testing apparatus including a. target, a'projecting system of optical elements forforming an image of the target upon the retina of a subjects eye, an observing systemof optical elements for-viewing the retina and the pupil of the subjects eye, a delimiting deviceyfor selecting the area of'thepupil through which light-entersthe eye, disposed so that an image thereof is substantially at an image of the pupil in light diverging from each point in the target, and a rotatable reversing member which is'common to the projecting and observing systems-and which has an odd number not less than three andnot greaterthan five: of refie'ctive faces, elements of the observing and projecting systems on the target side of said rotatable reversing member being disposed upon an axis whichis parallel to but offset with respect to an axis upon which the member and elements on the eye side of the member are disposed.-

Sight-testing apparatus including a target, a projecting system for forming an'image ofthe target upon the retina of a subjects eye, an observing'system for viewing the retina and the pupil of the subjects'eye, a rotatable reversing member, common to said projecting and observingsystems, having an odd number not less than three and not greater than five of reflective faces, and interlocked with said reversing member a rotatable pointer disposed substantially at an image of the pupil.

"5.'=-Si g'ht-testing apparatus including a target, a'projecting system for forming an image of the target on the retina of a subjects eye, an ob-' serving-system for forming an image of the retina, and a rotatablereversing member common to the two systems having one of its refractive faces disposed nearsaid image of-the retina and inclined at a" small angle to the axis ofthe member, its

other refractivefacedisposed substantially normal to-the axis of the member, and having an odd number not less than three and not greater' than five fof reflective faces.

, 6'. Sightetesting apparatus including a target,

a sprojectingsystem for forming an image of the targetu'pon the retina of a'subjects' eye,an .ob.-,

serving system for viewing the retina and the pupil of the subjects eye, a delimiting'device, for selecting the area of the'pupil through which light enters the eye, disposed substantially at an image of the pupil in light diverging from the target, an opaque mask, which is the mirror image of said delimiting device, disposed substantially at an image of the pupil, and a rotatable reversing member, common to "the two systems having its refractive faces disposed substantially normal to its axis and having an odd number not less than three and not greater than five of reflective faces.

7. Sight-testing apparatus including a target,

a projecting system of optical elements for forming an image of the target upon the retina of a subjects eye, an observing systemof optical elements for forming an image of the retina, and a rotatable reversing p'rism,'common to the two systems, having one of its refractive'faces disposedhear said image of the retina and inclined atasmall angle to the axis of the prism, its other refractive face disposedsubstantially normal to the axis of the' prism, and having an odd number'not less than three and not greater than five of reflective faces, elements of the observing and projecting systems on the-target side of said rotatable reversing prism being disposed upon an axis which is parallel to, but offset with respect to an axis upon which the prism and elements on the eye'side of the prism are disposed; 7 RICHARD EDMUND REASON.

Certificate of Correction Patent No. 2,049,222. July 28, 1936.

RICHARD EDMUND REASON It is hereby certified that errors appear in the printed specification of the above numbered patent requiring correction as follows: Page 2, first column, line 9, for relimiting read delimiting; line 70, for lens read lenses, and second column, line 22, for abberation read aberration; line 51, strike out Fig. and insert instead the syllable are; line 52, for of first occurrence, read ofl; line 67, for 11 first occurrence, read 11; page 3, second column, line 17, for eleminated read eliminated; and that the said Letters Patent should be read with these corrections-therein that the same may conform to the record of the case in the Patent Ofiice.

Signed and sealed this 6th day of October, A. D. 1936.

[SEAL] HENRY VAN ARSDALE,

Acting Commissioner of Patents. 

